Oh, I know you probably have heard of Augmented Reality, but you have never experienced it in a way that is trully easy to use and that really feels... real. Here is a small demo from Total Immersion, a company that uses image recognition software to add computer generated interactive objects to real time videos.

As far as I understand it, they plot some points on the image, they store its contents and the software can recognize it even skewed or warped or even partially visible. In the video you will see how this applies to a classic green-screen-like board that the demonstrator can actually move around, then how it applies to an image on a t-shirt and then, finally, an interaction between computer generated and real objects.